1. Field of the Invention
The present invention relates to a method and to a circuit arrangement for activating a control device.
2. Description of the Related Art
Control devices are used, for example, in motor vehicles for controlling and regulating components and sequences. For the activation of control devices, circuit arrangements are used via which, for example, a supply voltage is provided. It must be noted that control devices must be protected against polarity reversal and overvoltage, for example, triggered by a load drop or load dump. Furthermore, a so-called main relay is provided for operating a control device which is to ensure a low quiescent current consumption.
It is necessary to protect electronic circuits against high negative current pulses, for example Pulse 1 from ISO 7637. This was previously achieved with high capacitances, for example, electrolytic capacitors in the on-board electrical system or inverse-polarity protection diodes. However, it must be taken into account that a high capacitance results in high costs. Moreover, a classic inverse-polarity protection diode causes a considerable voltage drop. This voltage drop of an inverse-polarity protection diode may be reduced by active inverse-polarity protection such as described in the German patent application publication DE 10 2009 029 514 A1.
The above-mentioned publication describes a circuit arrangement including a first field-effect transistor, a second field-effect transistor, a comparator, a suppressor diode and a trigger switch which serves to activate a control device. The comparator compares the voltage intended for activating the control device to a threshold voltage and, using a control unit, sets a clocked operation of the first field-effect transistor if the threshold voltage is exceeded. The described circuit arrangement fulfills the functions of an internal electronic main relay, a load dump protection, an inverse-polarity protection and a plug contact protection during plugging while energized.
The disadvantage of active inverse-polarity protection is, however, that this protection is no longer effective in the case of quick, negative pulses or that the used inverse-polarity protection field-effect transistor ruptures (Avalanche effect). This means that, for example, a protection with sufficient capacitances is necessary. A current may flow in the case of a negative pulse via the substrate of integrated semiconductor circuits, for example, ASICs, which are connected between ground and supply voltage. If this current is too high, it may result in malfunctions or even destruction.